Sparks and electrical breakdowns in composite materials are presently the subject of extensive research and development. Sparking by composite materials is a complex phenomenon, and may result from different sources or modes, including high voltage loads and high thermal loads. The ever-increasing use of composite materials is driving the need to better analyze and discriminate between the different modes that cause sparking of composite materials.
In the aerospace industry, for example, the use of composite materials is being extended to many aircraft components that have historically been fabricated using metals, such as airframes and fuselage structures. The possibility of lightning strikes of such composite components increases the potential for a range of events, including ejection of hot particles from fastened joints where currents pass between metal fasteners and other structural elements, edge glow (or edge incandescence) at cut edges of composite spars and stringers, electrical breakdown between plies within composite materials that results in volatilization of resin material and forced expulsion of this material through formation of cracks, hot soot and gas plumes resulting from such expulsion, and electrical breakdown between metal fastener pins and other metal parts, such as fastener collars, shear ties, or metallic ribs. Possible effects of the heating due to a lightning strike of a composite component may include delamination of the structural plies, microscopic cracks in the composite, and the formation of small pyrolized or mass-depleted regions.
The phenomena commonly referred to as edge glow has also been referred to by other names, including surface glow, edge incandescence, edge sparking, and composite sparking. These names all reflect aspects of things known or suspected about the phenomena, and some may be good descriptions in some circumstances and not in others. In the past, edge glow has been detected by traditional Polaroid photographic methods such as that described in Aerospace Recommended Practice (ARP) 5416. However, the different light output of edge glow, as compared with that from electrical breakdown arcs, will mean that the film could respond differently even for equivalent amounts of energy. The threshold for detection of electrical breakdown arcs by the method of ARP5416 is approximately 200 microjoules.
Relatively little is known about edge glow or other composite sparking sources. Unknowns include the energy, temperature, and duration of edge and surface events, how event characteristics will impact the ability to detect light on film, the degree to which these events are driven by interply voltage arcs that occur at exposed edges, the size and composition of emitted material, the depths from which edge glow originates, and the role of surface condition.
Due to the potential importance of the composite material sparking phenomenon, improved methods and systems for analyzing and discriminating between various modes of composite sparking would have utility. Of particular importance to the aerospace industry are methods and systems that can be implemented easily for the large number and variety of tests that are needed to support aircraft certification by governmental authorities.